In the op-ed, Warren referenced Gorsuch's keynote address during a luncheon he attended at Trump International Hotel.

"That arrangement was bad enough on its own," Warren wrote.

ADVERTISEMENT

"But there was another potential conflict of interest created by Justice Gorsuch’s speaking engagement—and it highlights the ongoing ethical issues that threaten the credibility of our nation’s highest court."

Warren said on the same morning Gorsuch spoke at the luncheon, the Supreme Court said it would hear a case that "will determine whether public sector units—which represent teachers, nurses, firefighters and police in states and cities across the country—can collect fees from all employees in the workplaces they represent."

Gorsuch is expected to "deliver the court's deciding vote to strip unions of this ability," she wrote, adding that the decision would "seriously undercut workers' freedom" to fight for high wages and better working conditions.

"The Charles Koch Foundation is dedicated to promoting limited government, free markets and weaker unions; and the Bradley Foundation has worked for decades to, in their own words, 'reduce the size and power of public sector unions,' " she added.

Warren said the Code of Conduct for United States Judges requires judges to recuse themselves if there is a conflict of interest — but that doesn't apply to Supreme Court justices.

"In fact, Supreme Court justices are the only federal judges who are not bound by a formal code of conduct," she wrote.

"It is time to begin rebuilding American’s confidence in the court by establishing a formal code of conduct."

Warren said she is co-sponsoring a bill that would "require the Supreme Court to adopt an ethical code."

"Federal judges are not supposed to be politicians or advocates. They are supposed to rise above the political winds of the day and demonstrate a single-minded commitment to one promise: equal justice under law," she said.

"As judges of the nation’s highest court, it is time for Supreme Court justices to demonstrate that they can meet that standard."

Gorsuch last month gave the keynote address at an event held at the Trump International Hotel, despite blowback from outside groups concerned about the potential ethical conflicts created by appearing at a business owned by the president.